1. Field of the Invention
The present invention relates to a non-radiative dielectric waveguide used in a high-frequency band, such as a millimeter wave band, and more particularly to a non-radiative dielectric waveguide suitably used for a millimeter wave integrated circuit or the like. The invention also relates to a millimeter wave transmitting/receiving apparatus of non-radiative dielectric waveguide type, such as a millimeter wave integrated circuit or a millimeter wave radar module.
2. Description of the Related Art
A conventional non-radiative dielectric waveguide (hereafter referred to as an NRD guide) S11 is shown in FIG. 8. In the NRD guide S11 shown in FIG. 8, a dielectric strip 202 is interposed between a pair of parallel planar conductors 201 and 203 arranged at an interval d of λ/2 or below of a wavelength λ of an electromagnetic wave (high-frequency signal) propagating through the air at the usage frequency. This arrangement allows an electromagnetic wave to propagate along the dielectric strip 202. That is, this construction conforms to the principle of operation that a radiant wave is suppressed by the cut-off effects exerted by the parallel planar conductors 201 and 203.
The NRD guide S11 is operated in two electromagnetic-wave transmission modes, one of which is the LSM mode while the other is the LSE mode. In general, the LSM mode is put into wider use because of its small transmission loss. Moreover, as an NRD guide of another type, there is known an NRD guide S12 as shown in FIG. 9 that is provided with a dielectric strip 214 in a curved shape. In this construction, an electromagnetic wave is allowed to propagate easily in a curve, so that miniaturization of a millimeter wave integrated circuit or highly flexible circuit design can be implemented.
Note that, in FIGS. 8 and 9, the upper parallel planar conductor 203 is partially cut away and another parallel planar conductor 213 is indicated by a broken line, so that their inner portions come into view. Numerals 201 and 211 represent lower parallel planar conductors.
As materials for the dielectric strips 202 and 214 of the NRD guides S11 and S12, resin materials having a relative dielectric constant of 2 to 4 have conventionally been used, such as Teflon (Trademark of DuPont; polytetrafluoroethylene) or polystyrene, in view of its easy processability and lower loss.
However, in the NRD guide realized by using a dielectric strip formed of a conventionally-used dielectric material having a relative dielectric constant of 2 to 4, such as Teflon or polystyrene, there is a transmission loss at a curved portion (to be simply called bending loss) or a transmission loss in a strip conjugating portion is great. This makes it impossible to provide a sharp curved portion. Moreover, even in a case where a gently-curved portion is formed, its radius of curvature needs to be determined precisely. Further, a frequency band width in which transmission can be achieved with lower bending loss is insufficient, for example, made as narrow as about 1 to 2 GHz in the vicinity of 60 GHz. This is because, in the NRD guide S11 or S12 employing a dielectric material having a relative dielectric constant of 2 to 4, since the dispersion curves of the LSM mode and LSE mode are very close to each other, part of electromagnetic waves of the LSM mode is converted into LSE mode, which results in an increase in transmission loss.
There is also known an NRD guide employing ceramics having a relative dielectric constant of about 10, such as an alumina (Al2O3) ceramics, as a material for the dielectric strips 202 and 214. However, such an NRD guide cannot be used in a high-frequency band of not lower than 50 GHz without making the width of the dielectric strips 202 and 214 extremely narrow, and is thus impractical in terms of processability and mountability.
Moreover, in an NRD guide realized by using a dielectric strip formed of a conventionally-used resin material such as Teflon, the dielectric strip and the parallel planar conductor cannot be bonded to each other with ease. Consequently, the dielectric strip is positionally deviated due to vibration or difference in thermal expansion, which results in malfunction.
Further, as disclosed in Japanese Unexamined Patent Publication JP-A 57-166701 (1982), the sectional configuration of the dielectric strip is not limited to a rectangular shape, but is required to be geometrically symmetric with respect to the shape of the parallel planar conductor in accordance with the principle of operation. In this respect, in an NRD guide realized by using a dielectric strip formed of a conventionally-used resin material such as Teflon, the dielectric strip is positionally deviated and its symmetrical configuration becomes deformed due to vibration or difference in thermal expansion, which results in malfunction.
If the dielectric strip exhibits an unduly small relative density, the dielectric constant is smaller than the material property value. This makes it impossible to obtain intended transmission characteristics. Moreover, in this case, the open pore ratio is increased, and therefore impurities, which are generated during the process steps for the dielectric strip, are attached to the strip surface, and the strip surface adsorbs moisture due to humidity of atmosphere. This causes deterioration of the transmission characteristics. Further, during the process steps, the dielectric strip suffers from a burr or chip, which makes difficult formation of the symmetrical configuration. Still further, in a case where the dielectric strip is secured with adhesive, the adhesive exhibiting large dielectric loss finds its way into the open pore portion or chipping of the dielectric strip that is a cause of a decrease in density. This causes attenuation of high-frequency signals, which results in an undesirable increase in transmission loss.
Particularly, in a case where a plurality of dielectric strip portions are arranged at narrow spaces and electromagnetically coupled to one another so as to form a single set of a long dielectric strip, the intrusion of adhesive exhibiting large dielectric loss between the dielectric strip portions causes significant deterioration in transmission efficiency of high-frequency signals, or leads to generation of discontinuities in high-frequency signal transmission.